1. Field of the Invention
The present invention relates to an imaging lens for image reading used in an optical system of image-reading apparatus such as facsimile machine, copying apparatus, and image scanner; and an image-reading apparatus using the same. In particular, it relates to an image-reading imaging lens used for reducing or enlarging images, and an optical apparatus using the same.
2. Description of the Prior Art
It is basically required for an image-reading imaging lens used for a facsimile machine, copying apparatus, image scanner, or the like of a type which forms a reduced or enlarged original image on an imaging device such as CCD to have high resolution, large marginal light quantity, and low distortion at the imaging magnification used. As those satisfying these requirements, there have been provided Gaussian type lenses of a six-lens element configuration (e.g., the one disclosed in Japanese Unexamined Patent Publication No. 59-90810) and Xenotar type lenses of a five-lens element configuration (e.g., the one disclosed in Japanese Unexamined Patent Publication No. 9-127414).
In addition to these requirements, there have recently been demands for reducing the weight of lens systems and accordingly the weight of the optical apparatus as a whole, and cutting down the manufacturing cost.
Known as an imaging lens which can respond to such demands is the one disclosed in commonly-assigned Japanese Unexamined Patent Publication No. 7-104185 in which the number of lens elements is reduced to 4.
Though image surface curvature and distortion are corrected well in the imaging lens of a four-lens element configuration disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 7-104185, its spherical aberration is somewhat large. As a consequence, though it has a sufficient resolution as an imaging lens used for an image-reading apparatus having a relatively low number of resolution lines (about 400 dpi or less on the original side), its resolution may be insufficient as an imaging lens used for an image-reading apparatus having a higher number of resolution lines (about 600 dpi or higher on the original side).